Attitude control of satellites is conventionally achieved by discharge of reaction mass to surrounding space from thrusters oriented such that the reaction force from the discharge rotates the satellite in the desired direction. The motion is stopped by thrust in the opposite direction. The thrusters are controlled by either on-board computer or from the ground to achieve the desired result. Attitude control is, of course, very important to assure that cameras, antennas and other sensing devices on board, as well as sunshades, solar panels, etc., remain continuously aimed in the desired directions to perform their functions.
In accordance with the conventional prior art, a reservoir of suitable fluid in liquid form must be stored on board and launched with the satellite, thereby increasing its launch weight. During use, this reservoir is gradually exhausted. When the reaction mass has been used up, attitude control of the satellite is lost. Typical of this prior art are the National Oceanic and Atmospheric Administration's GOES satellites with their external thrusters.
Another prior art system is disclosed in U.S. Pat. No. 3,097,818 to Heller wherein a reservoir of water is stored on board the satellite. A circular conduit is provided which is divided into two semi-circular conduit sections by a pair of vapor generators. These may be selectively heated to generate steam and direct the vapor into one or the other section where, when it reaches the other generator, it is condensed back to water again. The vapor, travelling in a circular path, has a certain angular momentum. The angular momentum of the fluid in one direction results in the satellite acquiring angular momentum in the opposite direction, thus changing its attitude. The motion is stopped by causing vapor flow in the opposite sense. If the effect thus produced on the attitude of the vehicle is insufficient, reaction mass can be discharged into space from external thrusters to supplement the action of the internal system.
It is apparent that problems exist with the Heller system. In the first place, a substantial reservoir of water is needed which must be in the vehicle at launch, thus increasing its weight and correspondingly decreasing the available useful payload. For use, the water must be heated to the vapor state and recondensed after use. This leads to slow response and problems of overshoot as well as high demand for electrical power. Further, a system for use in space requiring storage, vaporization and condensation of water for its success must, of necessity, be complex mechanically and electrically, presenting physical problems to overcome effects of weightlessness, enormous temperature changes etc., with corresponding likelihood of failure. In a weightless environment, storage and flow of liquid present problems especially difficult to overcome. Because Heller provides only low reaction power with rapid saturation of his semi-circular conduit sections, errors in attitude correction tend to be cumulative, requiring reaction mass from time to time to be discharged into space through external thrusters to compensate for the inadequate initial thrust, whereby the reservoir is sooner exhausted.
A somewhat similar mass transfer arrangement is disclosed in the Miksch U.S. Pat. No. 3,091,417 which also employs reservoirs of liquid, water or alcohol, to be vaporized by heaters. He too contemplates loss of fluid through thrusters discharging to space when needed. Miksch also provides slow response with overshoot problems. Again errors are cumulative, external thrusters being required to compensate for the same. His heaters likewise have considerable demand for electricity. To the extent that Miksch transfers a mass of liquid, he might as well be shifting a mass of solid with attendant limitation on thrust effect and increased error accumulation.
With the foregoing in mind, it is the primary object of the invention to provide a novel method and apparatus for satellite attitude control employing the principle of conservation of angular momentum but requiring the use of only light-weight components and a reservoir of gas instead of liquid with no loss of reaction mass in use and immune to the above-described saturation effect and error accumulation.